1. Field of the Invention
The present invention relates to an apparatus and method for generating a SRS transmit antenna pattern in an uplink wireless telecommunications system using multiple antennas and Sounding Reference Signal (SRS) hopping.
2. Description of the Related Art
In a wireless telecommunications system, the multiple antenna technique is used as one method for improving uplink performance. As a representative example, the Long Term Evolution (LTE) which is a next generation mobile communication system of the asynchronous cellular mobile communication standard group 3rd Generation Partnership Project (3GGP) also applies an antenna selective transmission diversity in Single Carrier Frequency Division Multiple Access (SC-FDMA) based uplink such that the performance can be improved through a space-diversity gain in an uplink.
Moreover, a terminal transmits SRS in order that a base station can acquire information of the uplink. The base station receives the SRS and obtains channel state information of the uplink band and, based on the information, performs frequency selective scheduling, power control, timing estimation and Modulation and Coding Scheme (MCS) level selection. Particularly, in the case where the terminal uses the antenna selective transmission diversity method, the base station selects an antenna having the best channel state among the uplink channel states measured from SRS transmitted from each antenna of the terminal. The terminal obtains a diversity gain by performing an uplink transmission through the selected antenna. To perform the above processes, the base station needs to determine the channel state of the entire band to which uplink data is transmitted for each terminal antenna. This becomes possible when the terminal transmits SRS throughout the uplink data transmission bandwidth for each antenna.
FIG. 1 illustrates an example of LTE uplink transfer structure. As shown in FIG. 1, a subframe 100 having the length 1 ms, which is a base unit of LTE uplink transfer, is comprised of two 0.5 ms slots 101. Assuming that the Cyclic Prefix (CP) has a usual length, each slot is comprised of seven symbols 102 while one symbol corresponds to one SC-FDMA symbol. A Resource Block (RB) 103 is a resource allocation unit corresponding to twelve subcarriers in a frequency domain, and one slot in a time domain. The structure of the uplink of the LTE is classified into a data region 104 and a control region 105. The data region is a series of communications resource including data such as voice data, packet data transmitted to each terminal, and corresponds to the resources except for the control region within the subframe. The control region is a series of communications resources including a downlink channel quality report from each terminal, a reception ACKnowledgement/Negative ACKnowledgement (ACK/NACK) for the downlink signal, and an uplink scheduling request.
As shown in FIG. 1, the time when the SRS can be transmitted within one subframe is an SC-FDMA symbol duration which is the final duration while the SRS is transmitted through a data transmission band on a frequency basis. The SRSs of various terminals transmitted through the final SC-FDMA of the same subframe can be classified according to the location of frequency. Moreover, the SRS is comprised of a Constant Amplitude Zero Auto Correlation (CAZAC) sequence, and SRSs transmitted from various terminals are a CAZAC sequence which has a different cyclic shift value. Each of the CAZAC sequences generated through a cyclic shift from one CAZAC sequence has the correlation value of zero with respect to the sequences having a different cyclic shift value. By utilizing such characteristics, the SRSs of the same frequency domain can be classified according to the CAZAC sequence cyclic shift value. The SRS of each terminal is allocated in the frequency domain based on the tree structure set in the base station. The terminal performs the SRS hopping to transmit the SRS to the entire uplink data transmission bandwidth in this tree structure.
FIG. 2 illustrates an example of an SRS allocation method for the tree structure set by a base station in a data transmission band corresponding to 40 RB on a frequency basis.
In this example, assuming that the level index of the tree structure is b, the most upper level (b=0) is comprised of one SRS BandWidth (BW) unit of 40 RB bandwidth. In the second level (b=1), two SRS BWs of 20 RB bandwidth are generated from the SRS BW of b=0 level. Therefore, two SRS BWs can exist in the whole data transmission band. In the third level (b=2), five 4 RB SRS BWs are generated from one 20 RB SRS BW of the very upper level (b=1) so that it has a structure where ten 4 RB SRS BWs exist within one level. The configuration of this tree structure can have a number of various levels, the SRS bandwidth and the number of SRS BWs per one level is set according to the setting of base station. The number of SRS BW of the level b generated from one SRS BW of the upper level is defined as Nb, and the index of SRS BW of Nb is defined as nb=[0, . . . , Nb−1]. In the example shown in FIG. 2, a user 1 200 is allocated to the first SRS BW (n1=0) among two SRS BWs having 20 RB bandwidth at b=1 level. A user 2 201 and a user 3 202 are allocated to the location of the first SRS BW (n2=0) and third SRS BW (n2=2) under the second 20 RB SRS BW. In this way, the conflict between terminal SRSs can be avoided and allocated based on the tree structure shown in the example.
FIG. 3 illustrates the SRS hopping transmission structure for the case in which a terminal does not use an antenna selective transmission diversity.
The nSRS is an SRS transmission point of time index, while having the value of 0, 1, 2, . . . . In this way, the SRS transmission for the entire uplink data transmission band becomes possible by performing the SRS hopping in the tree structure which is particular to a cell. In the case where the antenna selective transmission diversity method is supported in the LTE uplink system, the terminal transmits the SRS to each antenna so that base station may provide channel information to determine the terminal transmit antenna. Since the terminal performs the uplink transmission by always using a single antenna in the LTE system, the terminal alternately uses two antennas at the point in time of the SRS transmission while, at the same time, performing the SRS hopping shown in FIG. 3.
FIG. 4 illustrates an example of antenna pattern which transmits SRS when a terminal supports an antenna selective transmission diversity and performs an SRS hopping in the LTE system.
As shown FIG. 4, the conventional SRS transmit antenna pattern which uses a terminal antenna for the SRS transmission by turns result in transmitting the SRS to a frequency location which is restricted for each antenna. Accordingly, there is a problem in that each antenna cannot transmit an SRS to the entire uplink data transmission band. For example, assuming that two antennas of the user 1 allocated in FIG. 3 are Ant #0 300 and Ant #1 301, the Ant #0 always transmits the SRS to the left half of the uplink data band, while the Ant #1 transmits the SRS to the right half. Similar problems occur with respect to the user 2 and 3.